The doping of electrically active elements into semiconductors is at this time done almost exclusively by injecting the elements into the semiconductor material by means of instruments known as ion implanters. Ion implanters create highly controlled beams of the desired ions and drive these ions at relatively high energy into the semiconductor wafers so as to dope the wafers in a uniform manner, at a controlled depth and temperature, and with well-regulated dose strengths. The functional components of ion implanters are: a source, called the ion source, of the element to be implanted; an acceleration and control section whose function is to produce a beam of the homogeneous ions having the desired energy and intensity profile; a target system which holds the semi-conductor wafers which will be implanted; and a wafer handling system which loads fresh wafers into the target system and unloads the implanted ones. The wafers are generally much larger than the beam size so that it is necessary to also have a scanning system which either scans the beam over the wafers or mechanically scans the wafers across the beam. The present invention is directed towards the scanning function of the ion implantation system. It specifically concerns the technique for scanning the ion beam across wafers which are rotated or otherwise conveyed in front of the ion beam.
Some early proposals for scanning are given in Freeman, U.S. Pat. No. 3,689,766. There the beam is scanned by varying the energy of the beam and passing it through a magnetic field. Reduction in variation of the ion beam intensity was to be achieved by sensing the ion beam current and feeding back that signal to control the amplitude of the sweep voltage applied to the beam.
The scanning of the beam across the semi-conductor wafer has continued to be of concern throughout the evolution of the modern ion implanter. The problems of scanning get more severe as the wafers get larger. The 3 inch diameter wafers of a decade ago have been successively replaced with 4 inch, 5 inch, and 6 inch wafers; 8 inch diameter wafers are now becoming standard. A common requirement is to maintain spatial uniformity of the implanted ions over large surfaces even as the criteria for uniformity become more stringent; specificaions of &lt;1% non-uniformities of implant dose over the wafer area are now common. Examples of present day scanning techniques are given in Ryssel and Glawischnig, Ion Implantation Techniques, Springer-Verlag, pp. 1-20 (1982), see especially pages 4 and 9.